In the synthetic resin (plastic) industry, screw or worm presses have been developed for plastifying, masticating, displacing and extruding synthetic resin materials, particularly thermoplastics, and for mixing or otherwise treating these materials by mechanical and, where necessary, a combination of mechanical and thermal techniques.
The development of worm or screw presses for this purpose will be discussed in greater detail below, although initially some definitions are in order.
Thus, while reference may be made to extrusion presses herein, it should be noted that such presses are not limited to the production of extruded products since an extrusion press for preparing thermoplastic materials can also be employed in injection molding, blow molding and other applications common in the thermoplastics field. For example, the products may be extruded by a press through a die imparting the final shape or the extrusion press may simply prepare the feed for an injection-molding or blow-molding installation.
Reference will also be made herein to the screws or worms of such presses. For the purpose of this description, a screw or worm will be considered to be an elongated body having one or more helical ribs or spiral ribs which may have a screw thread profile or can deviate from such profile and which, when rotated, tends to advance a thermoplastic material through a worm housing from an inlet end to a discharge end. Each such rib will be considered a "flight" by analogy with the ribs of worm conveyors.
Naturally each flight will have a number of turns, the inter-turn spacing defining the pitch of the worm or screw. The outermost portion of each flight is referred to as the crest and low point between turns can be termed the root, the height or depth of the flight being equal to the radial distance between the root and the crest and being equal to (D-D'/2) where D is the outer diameter of the worm and D' is the root diameter. The flight height (or depth) can be represented by d.
As noted, the profile of the flight may be similar to that of a screw thread and, in practice, this profile will be generally trapezoidal. Each turn will therefore have two flanks which are inclined outwardly toward one another, the flank facing in the direction of advance of the material being termed the pressure flank because it provides the forward impetus to the material, the other flank of each turn being referred to as the trailing flank. Each flank includes an angle with a perpendicular to the axis of the worm, this angle being termed the flank angle.
For various configurations of worm presses, their applications, construction, drives and configurations, reference may be had to the publications discussed in the expanded consideration of the background below, and in the prior U.S. Pat. Nos. 3,913,897, 3,927,869, 3,929,322, 3,969,956 and 4,047,705. Reference may also be made to the publications in the files of these patents and in the classes to which these patents have been assigned in the Manual of Patent Classification.
Early use of screw or worm presses for polyolefins utilized single worm presses in which a worm or screw press having a cylindrical configuration primarily and one or more helical flights was driven to displace a granular or pulverulent polyolefin material, primarily polyethylene, from a hopper at the inlet end of the press to an outlet at the opposite end from which a continuous strand of the polyolefin emerged.
The flight depth, pitch and configuration were so selected that the mass was subjected to considerable shear with heat being generated by the friction of the shear energy and compression. It was possible to develop these parameters so that, without external heating, the polyolefin mass reached the desired plastification temperature.
Certain relationships were discovered at such early dates. For example, it was found that the stiffer the mass, the deeper the worm flight required. In practice, however, it was not possible to select the structural parameters which always resulted in the appropriate temperatures and thus a certain contribution to the energy was generally provided by controlled heating and by constriction of the flow of the material from the press or by increase of the residence time.
The viscosity of a polyolefin, moreover, drops sharply in the region of the extrusion temperature and hence the shearing energy likewise varies along the length of the path. Excess temperatures can develop close to the discharge end because of the viscosity drop and in practice it is required to cool the discharge end because the depth of the flight cannot be reduced beyond the level required for displacement of the material.
It has also been found that the handling of hard polyvinyl chloride (PVC) by single screw worm presses is only economical with very small machines of limited capacity. Hard PVC is not only stiffer than most polyolefins, it is also thermally unstable and reacts to overheating by adhesion to the worm, thermal decomposition and combustion with strong evolution of hydrogen chloride.
Because of this, double-screw worm presses have been developed, such presses having two intermeshing counter-rotating worms. These presses have increased the rate at which hard PVC could be treated. The presses had a worm length of 10 to 12 times the diameter and, while the flight height, output per revolution and rotational speed were small, only a simple drive was required for the two parallel worms and productivity was increased.
However, efforts to increase the speed, reduce the interaxial spacing of the worm, and the like to permit greater flight heights and outputs to be developed, fail for one or more reasons. For example, when the shafts of the worms were brought too closely together, extreme complex transmissions were required to drive the worms.
In addition, excessive length, deep flights and large pitches resulted in weakening the worms such that the weak points were not their drive gears or bearings, but rather the worm shaft or body of the worm itself.
The worms were incapable of effectively withstanding the high driving torque, enormous backpressure and non-uniform wear of the flights and the cylinder or housing in which the worms were rotated.
Indeed, the worms themselves were weakened by the need to provide cooling channels through the cores of the worms and breakage frequently resulted.
The depth of the flight was found to be limited by practical considerations, namely, the drive requirements and the strength and stability of the worm or its shaft.
The excess energy developed in the handling of PVC at high speeds at the discharge zone are extracted by cooling of the cylinder and the worm if thermal decomposition of the PVC is to be avoided. This is also a handicap since cooling of the cylinder may be inconvenient, but cooling of the worm is extremely complex and, where the desired temperatures cannot be maintained, such cooling must be supplemented by control actions such as a reduction in the throughput of the press. All of these requirements detrimentally influenced the output of the apparatus and increased production costs.
Thus the art has come forward with conical worms as a means of avoiding the disadvantages of the double-worm systems previously discussed. The double conical worm system permits the axes of the two worms to be inclined to one another and hence to have a progressively increasing spacing away from the discharge end of the press. The interaxial spacing in the region of the drive wheels and bearings can be comparatively large so that the drive system is relatively simple. The peripheral speeds at the upstream end of the worms, for a given angular velocity, are comparatively large while the peripheral speeds at the discharge ends are correspondingly small, thereby providing a more effective energy distribution, since most of the shear, compression and friction heat is developed at the upstream end while minimum shear energy is developed at the downstream end.
Nevertheless experience has shown that excess energy may still develop at the downstream or discharge end and that such presses have not fully eliminated all of the disadvantages of the earlier systems. Attempts have been made in this direction (see German patent document DE-OS No. 24 46 420 and Austrian Pat. No. 356,882) with only partial success.